Project Summary/Abstract Fc receptors (FcR) modulate intracellular signaling upon IgG binding in the effector cells in the immune system. We have previously found that the inhibitory FcR known as Fc?RIIB is also expressed in skeletal muscle microvascular endothelium, and that global Fc?RIIB null mice are protected from high-fat diet (HFD)-induced insulin resistance. To explore the role of endothelial Fc?RIIB in insulin metabolism, we generated the mouse strain lacking Fc?RIIB specifically in endothelial cells by crossing floxed Fc?RIIB (Fc?RIIBfl/fl) with VE Cadherin-Cre mice (Fc?RIIBfl/fl:VECad-Cre), and fed them with control diet or HFD. We found that Fc?RIIBfl/fl:VECad-Cre mice are protected from HFD-induced peripheral insulin resistance and from impairment of glucose delivery to the skeletal muscle. Furthermore, we discovered that IgG isolated from HFD-fed wild-type mice (HFD-IgG), but not IgG from control diet-fed mice (Con-IgG), induces insulin resistance when transferred into mice lacking endogenous IgG (B-/- mice) in an Fc?RIIB dependent manner, and that HFD-IgG is less sialylated in its Fc domain compared to Con-IgG. Treatment of wild-type mice with N-Acetyl-D-mannosamine (ManNAc) that increases protein sialylation ameliorated HFD-induced insulin resistance. Moreover, IgG isolated from obese type 2 diabetes mellitus (T2DM) patients, but not from non-T2DM subjects, induced insulin resistance in B-/- mice via Fc?RIIB. Based upon these novel findings, the overall goal of the proposed project is to determine how IgG sialylation is dysregulated in diet-induced obesity (DIO) and how endothelial Fc?RIIB contributes to the pathogenesis of DIO-related insulin resistance using both mouse models and cultured cells. Aim 1 will determine how HFD decreases sialylation of IgG, focusing on the process in B cells that modulates IgG sialylation. We will also determine how ManNAc prevents HFD-induced insulin resistance. Aim 2 will determine how endothelial Fc?RIIB mediates HFD-induced peripheral insulin resistance, testing the hypothesis that activation of endothelial Fc?RIIB by HFD-IgG initiates intracellular signaling that leads to attenuation of transendothelial transport of insulin. Aim 3 will determine whether IgG sialylation and its ability to induce insulin resistance in mice are associated with DIO-related insulin resistance in humans, using existing cohorts of subjects with a range of obesity and insulin sensitivity. Using a highly translational approach, we will test the novel concept that Fc?RIIB in endothelium and modification of IgG in B cells are critically involved in the pathogenesis of insulin resistance and T2DM. We anticipate that the new knowledge gained will lead to novel preventative and treatment measures to combat the insulin resistance that characterizes obesity and other chronic inflammatory conditions.